[fpmsyncd] Fpmsyncd Next Hop Table Enhancement (#2919)

What I did
Implementing code changes for sonic-net/SONiC#1425

Why I did it
add nexthop group feature to fpmsyncd.

How I verified it

enable/disable nexthop group feature
Klish will call REST API to configure feature next-hop-group enable.
FEATURE|nexthop_group will be created in CONFIG_DB
template zebra.conf.j2 will generate zebra.conf with fpm use-next-hop-groups if FEATURE|nexthop_group exists in CONFIG_DB. Else, it will generate zebra.conf with no fpm use-next-hop-groups (default behavior)
Do config save comman and write to /etc/sonic/config_db.json
restart SONiC: virsh reboot sonic-nhg
/etc/frr/zebra.conf has fpm use-next-hop-groups instead of no fpm use-next-hop-groups

Author: ntt-omw

fpmsyncd/fpmlink.cpp

@@ -281,6 +281,11 @@ void FpmLink::processFpmMessage(fpm_msg_hdr_t* hdr)
             /* EVPN Type5 Add route processing */
             processRawMsg(nl_hdr);
         }
+	else if(nl_hdr->nlmsg_type == RTM_NEWNEXTHOP || nl_hdr->nlmsg_type == RTM_DELNEXTHOP)
+        {
+            /* rtnl api dont support RTM_NEWNEXTHOP/RTM_DELNEXTHOP yet. Processing as raw message*/
+            processRawMsg(nl_hdr);
+        }
         else
         {
             NetDispatcher::getInstance().onNetlinkMessage(msg);
@@ -320,4 +325,4 @@ bool FpmLink::send(nlmsghdr* nl_hdr)
     }
 
     return true;
-}
+}